* Optimized for mobile.
* MVC structure support.
* Easier to port to other platforms (platforms like iOS, Android and BlackBerry). 90 - 95% of code is shared between different platforms.
* Can distribute the application with the interference of any app store.
* Extensive and well arranged API.
* Rich, customizable, and nearly native UI elements (list view, tab bar, button). All these can be customized with css.
* Full touch support for mobile browsers, with handling of touch events such as swipe, tap, and long press.
* Native packaging support for android and iOS.
* Local storage support.
* Data binding support.
* Offline support.
* Code can be written using any operating system and can be tested using a web server.
* Applications can be tested on chrome, safari, and IE10.
Room for Improvement:
* Scrolling needs some serious tweaking, specifically for android. There is a huge time lag while scrolling through a list.
* Lacks full access to native hardware, but if you want access to more hardware then you will have to combine your sencha application with PhoneGap.
* No support for blackberry. The sencha sdk beta tools provide support for native packaging for only android and iOS (even their team says they haven't tested much on blackberry).
* It's still in its developing phase and may take a while for it to fully mature.
* Animations run slower.
* No support for windows phone as well.
There are a number of options if you are looking to make a mobile web application, like sencha, jqtouch, etc. If you are seeking to make an application which consists of a large number of views with data requests and a large number of components in your view, I would suggest to go for sencha as it provides an MVC structure approach and their API, which is very well arranged. In my own experience, I have found the sencha support team to be very active on their forums. Their website also has numerous tutorials in their "learn" section. I would highly recommend sencha mobile application frameworks for either cross platform or standalone web apps. My own experience says that it's better than any other out there.